i 9 o 4 ] MAC DOUGAL— DELTA AND DESERT VEGETATION 47 



THE DELTA. 



The expedition east loose from the shore at Yuma at noon on 

 January 28, and within a short distance below the sand bluffs on 

 either hand curved away from the stream, and we were fairly in the 

 great delta which extends from this point to the Gulf of California, 

 a distance of about i40 km ; while the coastal plains on the western side 

 of the Gulf embrace mud flats that constitute an actual extension 

 of the delta so km further. This delta probably offers more varied and 

 striking features of natural history than any other watercourse in 

 North America. The river which has formed it rises in the perpetual 

 snows of Utah, Wyoming, and Colorado, and runs 25oo km , chiefly 

 through arid regions, before it empties into the upper end of the sub- 

 tropical Gulf, into which it carries sixty million tons of sediment 

 yearly, building up the delta and extending it seaward at a rate visible 

 to common observation within a single lifetime. 8 Numerous wit- 

 nesses among the Cocopa Indians, Mexicans, and river men are 

 agreed that the various distinct associations of plants characterized 

 by salt grass, willow, and poplar, have advanced about i2-i4 kra to the 

 southward during the last fifty years. 



The portion of the delta near the present course of the river con- 

 sists of an alluvial plain, not more than 4 m above the low-water 

 mark, subject to constant bank erosion, shifting, and remaking of the 

 soil, cut in all directions by old channels existing as bayous and 

 sloughs, and flooded at high water in May, June, and July. Almost 

 pure formations of willow and poplar (Populus mexicana) cover 

 many square kilometers and furnish food for thousands of beavers 

 that burrow in the banks. The poplar is thickly infested with a 

 mistletoe (Phoradendron) , and fungal parasites are abundant. 

 Large areas are occupied by the arrow-weed (Pluchea sericea), and 

 mesquite (Prosopis velutinea), and the screw-bean or "tornilla" 

 (P. pubescens). Two or three species of Atriplex are also to be 

 found in sections in which the action of the water prevents the estab- 

 lishment of the woody perennials of greater size. In the upper part 

 of the delta a cane (Phragmites) fringes the channel, and its closely 

 interwoven roots act materially in preventing erosion of the banks. 



8 Forbes, R. H., The Colorado river of the west. Univ. of Ariz. Monthly 6:112. 

 • 1904. 



